An ATM network needs to support certain service categories defined by ATM Forum standards. Among these service categories are constant bit rate (CBR), real-time variable bit rate (rt-VBR), non-real-time variable bit rate (nrt-VBR), available bit rate (ABR), and unspecified bit rate (UBR). The problem with VBR traffic is that there is great variation in the bit rate of each connection, yet the network has to guarantee a certain maximum cell loss ratio (CLR) for each connection.
Each ATM network node needs a connection admission control (CAC) procedure to determine whether a new connection can be admitted. If too many connections are admitted, the network cannot provide the agreed CLR, since it only has a limited capacity. If too few connections are admitted, the network capacity is not used efficiently. The challenge of CAC is to calculate how many connections the network node can have without violating the CLR constraint of the connections.
Telecommunications companies and universities have investigated the CAC problem for several years. Basic CAC algorithms which can be used for the CAC decision are based on two traffic parameters, i.e. the peak cell rate (PCR) and the sustainable cell rate (SCR, also known as the average cell rate).
In these algorithms a worst-case scenario is assumed, which means that all traffic sources are assumed to be on-off sources, either transmitting at the PCR or not transmitting at all. The probability p that a source is transmitting at any moment in time is given by p=SCR/PCR.
Furthermore, these algorithms assume that cells cannot be buffered to any significant extent. Only cell-scale buffers, which prevent cell loss if a few sources send their cells simultaneously, are included and in the burst scale the ATM Switch is assumed bufferless. This is a realistic assumption for real-time VBR traffic, which cannot be buffered anyway due to its stringent delay requirements.
According to a so-called PCR allocation, bandwidth (which is a common term for transmission capacity) is reserved at the peak rate (PCR) of a connection. However, this is clearly an overly conservative estimate and results in very low bandwidth utilization.
According to a so-called large deviations approximation described in “Broadband network teletraffic: performance evaluation and design of broadband multiservice networks; final report of action COST 242” by James Roberts et al., Springer-Verlag, Berlin, 1996, a theory is provided which allows to calculate very accurate values for very small CLRs (10−4 . . . 10−15).
Other approximations for finding the bandwidth requirement of a set of connections have been proposed by e.g. Guerin, Lindberger and Kalevi Kilkki.
However, the known solutions give good results but require so much computation that they might no be sufficiently fast in real-time CAC decisions. The large deviation theory is an example of this. Other solutions are a lot faster but relatively inaccurate in terms of bandwidth utilization. The PCR allocation is an extreme example of this.